Process and device for monitoring the supply of substitution fluid during an extracorporeal treatment of blood

ABSTRACT

The invention relates to a process for monitoring the supply of substitution fluid upstream or downstream of a dialyser or filter arranged in an extracorporeal blood stream. One embodiment provides, for the detection of predilution or postdilution, for measuring the pressure in the blood stream downstream of the dialyser or filter, predilution or postdilution being recognized on the basis of the change in pressure following the shutting off and/or starting up of the substituate pump provided for conveying the substitution fluid. Another embodiment provides for recognizing predilution or postdilution on the basis of the comparison of the oscillating pressure signal attributable to the substituate pump to a characteristic reference signal. The characteristic reference signal to which the pressure signal of the substituate pump is compared is preferably the oscillating pressure signal of a blood pump arranged in the blood stream for conveying the blood upstream of the dialyser or filter. In addition, the invention relates to an extracorporeal blood treatment device with a facility for detecting predilution or postdilution, which device operates according to the processes detailed above.

FIELD OF THE INVENTION

The invention relates to a process for monitoring the supply ofsubstitution fluid for a device for the extracorporeal treatment ofblood with an extracorporeal blood stream which includes a first chamberof a dialyser or filter divided by a membrane into the first chamber anda second chamber, and a fluid system which includes the second chamberof the dialyser or filter. In addition, the invention relates to such adevice for the extracorporeal treatment of blood using a facility forthe detection of the supply of substitution fluid upstream or downstreamof the dialyser or filter.

BACKGROUND

For the removal of substances normally contained in the urine and forthe withdrawal of fluid, different processes are used in the case ofchronic kidney failure for the extracorporeal treatment and/orpurification of the blood. In hemodialysis, the blood of the patient ispurified outside the body in a dialyser. The dialyser is equipped with ablood chamber and a dialysis fluid chamber which are separated by asemipermeable membrane. During the treatment, the blood of the patientflows through the blood chamber. In order to effectively removesubstances normally contained in the urine from the blood, freshdialysis fluid flows continuously through the dialysis fluid chamber.

While, during hemodialysis (HD), the transportation of the smallmolecular substances through the membrane is determined mainly by thedifferences in concentration (diffusion) between the dialysis fluid andthe blood, substances dissolved in the plasma water, in particularsubstances with a higher molecular weight, are effectively removedduring hemofiltration (HF) by a high fluid stream (convection) throughthe membrane of the dialyser. During hemofiltration, the dialyser actsas a filter. Hemodiafiltration (HDF) is a combination of the twomethods.

In hemo(dia)filtration part of the serum withdrawn through the membraneis replaced by a sterile substitution fluid which is passed to theextracorporeal blood stream either upstream of the dialyser ordownstream of the dialyser. The supply of substitution fluid upstream ofthe dialyser is also referred to as predilution, and the supplydownstream of the dialyser is also referred to as postdilution.

Devices for hemo(dia)filtration are known in the case of which thedialysis fluid is produced online from fresh water and concentrations,and the substitution fluid is produced online from the dialysis fluid.

In the known hemo(dia)filtration devices, the substitution fluid ispassed to the extracorporeal blood stream from the fluid system of themachine via a substitution fluid line. In the case of predilution, thesubstitution fluid line leads to a connecting site on the arterial bloodline upstream of the dialyser or filter, whereas during postdilution thesubstitution fluid line leads to a connecting site on the venous bloodline downstream of the dialyser. The substitution fluid line generallyexhibits a connector by means of which it can be connected either to thevenous or the arterial blood line. To interrupt the fluid supply, aclamp or the like is provided on the substitution fluid line. Such ahemo(dia)filtration device is known, e.g., from EP-A-0 189 561.

Monitoring of the treatment of the blood presupposes the knowledge as towhether the substitution fluid is passed to the extracorporeal bloodstream upstream or downstream of the dialyser or filter. Predilution andpostdilution, for example, play a part as regards the online clearancemeasurement (OCM) based on a conductivity measurement since theconductivity of the dialysis fluid downstream of the dialyser depends onwhether a predilution or postdilution takes place.

EP-A-1 348 458 A1 describes a process and a device for monitoring thesupply of substitution fluid for an extracorporeal blood treatmentdevice. For the detection of the supply of substitution fluid upstreamor downstream of the dialyser or filter, the period of operation of thepressure waves of a substituate pump arranged in the substitution fluidline is measured. The supply of substitution fluid upstream ordownstream of the dialyser or filter is recognized on the basis of themeasurement of the operating time. The known process presupposes the useof a substituate pump producing pressure waves.

DE 101 15 991 C1 describes a device for detecting stenoses in a hosesystem. The printed document proposes to draw the conclusion of thepresence of a stenosis (narrow site) in the hose system in the case of achange in the frequency spectrum of an oscillating pressure signalspreading in the hose system. The operating principle of the knowndevice is based on the fact that the cause of the change in the dynamicbehaviour of the hose system is the compliance of the hose system, i.e.the elastic yield under pressure.

SUMMARY OF THE INVENTION

An object of the invention is to indicate a process which allows therecognition of predilution or postdilution with a high level ofreliability. In addition, it is an object of the invention to indicate adevice for the extracorporeal treatment of blood, wherein the device isequipped with a facility by means of which predilution and postdilutioncan be reliably recognized.

Different embodiments of the invention are provided; however, theembodiments are based on measuring the pressure in the fluid systemdownstream of the dialyser or filter.

In one embodiment of the invention, the substituate pump conveying thesubstitution fluid is shut off and/or started up. The inventor hasrecognized that the change in the pressure following the shutting offand/or starting up of the substituate pump takes a characteristiccourse. The supply of substitution fluid upstream or downstream of thedialyser or filter is recognized on the basis of the change in pressurefollowing the shutting off or starting up of the substituate pump. Asudden increase in pressure and/or pressure drop is a parametercharacteristic of the change in pressure.

Basically, the recognition can take place only following a change in thepressure after the substituate pump has been shut off or started up. Agreater reliability, however, can be achieved if the recognition of thepredilution or postdilution takes place on the basis of the change inpressure both during the shutting down and the starting up of thesubstituate pump or vice versa.

In a preferred embodiment of the invention which allows the recognitionof predilution and postdilution with a particularly high reliabilitylevel, the substituate pump which has been started up is shut off andafter a predetermined time interval started up again, the pressure beingmeasured downstream of the dialyser or filter. A supply of substitutionfluid upstream or downstream of the dialyser or filter is recognized onthe basis of the change between a previous increase in pressure and asubsequent pressure drop or between a previous pressure drop and asubsequent increase in pressure.

If a sequence of pressure increase and pressure decrease is detected,the conclusion is drawn that the supply of substitution fluid takesplace downstream of the dialyser or filter, whereas the conclusion isdrawn that the supply of substitution fluid takes place upstream of thedialyser or filter if a sequence of a pressure decrease and an increasein pressure is detected.

One advantage in this embodiment of the invention consists of nopressure pulses being evaluated which propagate themselves in the bloodstream. Consequently, it is not necessary to provide a substituate pumpproducing an oscillating pressure pulse in order to be able todifferentiate between post-dilutive and pre-dilutive substitution.

To suppress interference signals, the pressure signal is preferablyfiltered with a low-pass filter.

The characteristic pulse sequence following the stop and start of thesubstituate pump can take place on the basis of a comparison of thepressure signal in the blood stream downstream of the dialyser or filterwith an upper and lower limit value which is characteristic of anincrease in pressure or pressure drop attributable to predilution orpostdilution.

The facility for the detection of the supply of substitution fluidupstream or downstream of the dialyser or filter of the extracorporealblood treatment device according to an embodiment of the invention isequipped with a control unit for shutting off and/or starting up thesubstituate pump, a measuring unit for measuring the pressure in theblood stream downstream of the dialyser or filter and an evaluationunit. The evaluation unit is designed such that a supply of substitutionfluid upstream or downstream of the dialyser or filter is recognized onthe basis of the change in the venous pressure after shutting downand/or starting up the substituate pump.

The measuring unit for measuring the venous pressure is preferablyequipped with a venous pressure sensor and a low-pass filter connectedto the signal output end of the pressure sensor.

The evaluation unit is preferably equipped with a comparator whichcompares the output signal of the pressure sensor for the detection ofan increase in pressure and/or pressure drop with a predetermined upperand/or lower limit value.

The process according to an embodiment of the invention and the deviceaccording to an embodiment of the invention are advantageously used fordetermining the dialysis dosage using the so-called online clearancemonitoring (OCM), and in the case of a substituate supply as a functionof the blood flow. In addition, the process and the device provide fordecision-making assistance regarding all relevant parameters relating todialysis if a distinction is necessary between the post-dilutive and thepre-dilutive substituate administration. The process and the device mayalso be used for the determination of the blood flow, the determinationof the shunt or fistula recirculation, the monitoring of the relativeblood volume and/or hematocrit, the determination of the filling volumesof the dialyser or filter and the recognition of the venous needle type.

Most of the components necessary for the detection facility aregenerally already present in the known blood treatment devices. Forexample, it is possible to make use of the venous pressure sensor formeasuring the pressure in the blood stream downstream of the dialyser orfilter. A microprocessor control is also available. Consequently, theexpenditure for the present invention in terms of equipment isrelatively low.

Another embodiment of the invention presupposes the use of a substituatepump producing pressure waves. The supply of substitution fluid upstreamor downstream of the dialyser or filter is recognized by means of acharacteristic reference signal on the basis of the comparison of theoscillating pressure signal which propagates itself within the bloodstream and is attributable to the substituate pump. The process and thedevice according to this embodiment of the invention are based on thefact that the frequency spectrum of the oscillating pressure signaldepends on whether the pressure signal propagates itself via thedialyser in the case of predilution or not via the dialyser in the caseof postdilution. Also, the amplitude of the pressure pulse changes as afunction of predilution or postdilution.

In the case of the known blood treatment devices, the oscillatingpressure signal of the substituate pump which propagates itself withinthe extracorporeal blood stream is superimposed by further pressuresignals which are attributable, for example, to the blood pump arrangedwithin the blood stream upstream of the dialyser or filter or tofacilities arranged within the fluid system which include, for example,the concentrate pump, the ultrafiltration pump or the balance chamber.For this reason, the pressure signal attributable to the substituatepump is obtained from the pressure signal measured in the blood stream.To avoid errors of measurement, the dialyser or filter can also beseparated from the fluid system.

A preferred embodiment of the invention which allows detection of thesupply of substitution fluid with a particularly high reliability levelprovides, as characteristic reference signal, the signal of the bloodpump arranged in the extracorporeal blood stream upstream of thedialyser or filter, which pump also produces oscillating pressuresignals. This embodiment is based on the assumption that, duringpredilution, the oscillation of the pressure signals of the blood pumpand the substituate pump which propagate themselves through the dialyserand the hose system, differs only insignificantly. In the case ofpostdilution, on the other hand, the oscillation of the pressure signalsof the substituate pump and the blood pump differs substantially sincethe pressure signals of the substituate pump do not propagate themselvesvia the dialyser. In this way, not only a relative but also an absolutedistinction characteristic is obtained between predilution andpostdilution.

To obtain the pressure signals of the blood pump and the substituatepump, a Fourier analysis of the pressure signal measured downstream ofthe dialyser or filter is preferably carried out. From the pressuresignals of the substituate pump and the blood pump, the amplitudes of atleast two harmonic components are preferably determined. At least onefirst evaluation factor is determined from at least one quotient of atleast one harmonic component of higher order and at least one harmoniccomponent of lower order of the substituate pump, and at least a secondevaluation factor is determined from at least one quotient of at leastone harmonic component of higher order and at least one harmoniccomponent of lower order of the blood pump.

In practice, it has proved to be sufficient if, by way of the Fouriertransformation, the coefficients of the second or higher harmonic of thepressure signal of the blood pump and/or sub stituate pump arestandardized with respect to the coefficient of the first harmonic ofthe pressure signal of the blood pump and/or substituate pump in orderto form the two evaluation factors.

The conclusion that a postdilution takes place is drawn if thedifference between the first evaluation factor and the second evaluationfactor is greater than a predetermined limit value whereas theconclusion that predilution takes place is drawn if the differencebetween the first evaluation factor and the second evaluation factor issmaller than the predetermined limit value.

The facility for the detection of the supply of substitution fluid tothe extracorporeal blood treatment device according to an embodiment ofthe invention is equipped with a measuring device for measuring thepressure in the blood stream downstream of the dialyser or filter and anevaluation unit which in turn is equipped with means for obtaining theoscillating pressure signal attributable to the substituate pump, meansfor comparing the oscillating pressure signal with a characteristicreference signal and means for detecting predilution or postdilution onthe basis of a comparison of the oscillating pressure signal of thesubstituate pump with the characteristic reference signal.

A distinction between predilution and postdilution can be made at thebeginning of the dialysis treatment alone by evaluating the pressuresignal of the substituate pump, but preferably in conjunction with thepressure signal of the blood pump.

Apart from the measuring unit for measuring the pressure in theextracorporeal blood stream downstream of the dialyser or filter whichis, in general, part of the known blood treatment devices, no furtherhardware components are required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly simplified diagrammatic representation of anembodiment of a device according to the invention for the extracorporealtreatment of blood with a facility for detecting predilution andpostdilution.

FIG. 2A shows a basic illustration of an example of the change of thepressure, over time, in the extracorporeal blood stream downstream ofthe dialyser or filter in the case of postdilution.

FIG. 2B shows a basic illustration of an example of the change, overtime, of the venous pressure in the case of predilution.

FIG. 3A shows the average venous pressure measured during an in vitroHDF dialysis treatment as a function of the time in the case ofpostdilution

FIG. 3B shows the average venous pressure in the case of predilution.

FIG. 4 is a greatly simplified diagrammatic representation of anotherembodiment of the extracorporeal blood treatment device according to theinvention.

FIG. 5A shows an example of the frequency spectrum of the pressure wavesin the extracorporeal blood stream which are attributable to thesubstituate pump and the blood pump and a table with standardizedcoefficients of the pressure signals of the blood pump and thesubstituate pump in the case of postdilution.

FIG. 5B shows an example of the frequency spectrum of the pressure wavesin the extracorporeal blood stream which are attributable to thesubstituate pump and the blood pump and a table with standardizedcoefficients of the pressure signals of the blood pump and thesubstituate pump in the case of predilution.

DETAILED DESCRIPTION

In the following, embodiments of the process according to the inventionand embodiments of the blood treatment device according to the inventionare explained in further detail with reference to the figures.

FIG. 1 shows a simplified diagrammatic representation of the essentialcomponents of a hemo(dia)filtration device together with a facility forthe detection of the supply of substitution fluid in the extracorporealblood stream upstream or downstream of the dialyser or filter. When wespeak of a dialyser in the following, this should be understood toinclude a filter.

The hemo(dia)filtration device is equipped with a dialyser 1 which isseparated by a semipermeable membrane 2 into a first chamber 3 throughwhich blood flows and a second chamber 4 through which a dialysis fluidflows. The first chamber 3 is operated in an extracorporeal blood stream5A whereas the second chamber 4 is operated in the fluid system 5B ofthe hemo(dia)filtration device.

The extracorporeal blood stream 5A comprises an arterial blood line 6which leads to the inlet 3 a of the blood chamber 3 and a venous bloodline 7 which leads from the outlet 3 b of the blood chamber 3 of thedialyser 1. To eliminate air bubbles, an arterial drip chamber 8 isinserted into the arterial blood line 6 and venous drip chamber 9 intothe venous blood line 7. The patient's blood is conveyed through theblood chamber of the dialyser by means of an arterial blood pump 10, inparticular a roller pump, which is arranged on the arterial blood line6.

The fluid system 5B comprises a dialysis fluid feed line 11 which leadsto the inlet 4 a of the dialysis fluid chamber 4 and a dialysis fluiddischarge line 12 which leads from the outlet 4 b of the dialysis fluidchamber 4 of the dialyser 1. Fresh dialysis fluid flows via the dialysisfluid feed line 11 from a dialysis fluid source, which is notillustrated, into the dialysis fluid chamber whilst the spent dialysisfluid is discharged from the dialysis fluid chamber via the dialysisfluid discharge line 12 to a drain line which is not illustrated. Thebalancing device which is generally provided in the hemo(dia)filtrationdevices for balancing fresh with spent dialysis fluid is not illustratedfor the sake of clarity. Also, additional facilities for purifying andrinsing the system are not illustrated.

The dialysis fluid supply line 11 comprises a first section 11 a whichleads to the inlet 13 a of a first chamber 13 of a sterile filter 16subdivided by a membrane 14 into the first chamber and a second chamber15, and a second section 11 b which leads from the outlet 13 b of thefirst chamber 13 of the filter 16 to the inlet 4 a of the dialysis fluidchamber 4.

During dialysis treatment, the dialysis fluid, for example, can besupplied from the fluid system 5B as substitution fluid via asubstitution fluid line 17 to the extracorporeal blood stream 5A. Thefluid line 17 is equipped at both ends with two line sections 17 a, 17b, 17 c, 17 d respectively. Line section 17 a is connected with thefirst outlet 15 a of the sterile filter 16 and line section 17 b isconnected with second outlet 15 b, whereas a connector 18 a, 18 b isconnected to line sections 17 c and 17 d. By means of the two connectors18 a, 18 b, the substitution fluid line 17 can be connected to aconnecting line 19 leading to the arterial drip chamber 8 and/or to aconnecting line 20 leading to the venous drip chamber 9. The connectinglines 19, 20 are equipped for this purpose with correspondingconnections 19 a, 20 a. Hose clamps 17 e, 17 f are fitted to the hoseline sections 17 c, 17 d, by means of which clamps a fluid connectioncan optionally be established to the arterial or venous drip chamber 8,9. It is also possible for the substitution fluid line 17 to beconnected with both connecting lines 19, 20 and both hose clamps 17 e,17 f to be open.

To clamp the substitution fluid line 17 shut, a shut-off device 21, e.g.a hose clamp, is provided downstream of the sterile filter 16, whichclamp is preferably operated electromagnetically. The substitution fluidis conveyed by means of an occlusion pump, in particular a roller pump22, into which the substitution fluid line 17 is inserted. Such rollerpumps are part of the state of the art. They are equipped with severalrolls 22 a, 22 b by means of which the cross-section of the hose linefor conveying the fluid can be reduced. In this way, pressure waves areformed which are capable of propagating themselves in both directionsthrough the substitution fluid line. In should be noted that anocclusion pump producing pressure waves is not required as substituatepump in the case of this embodiment of the blood treatment deviceaccording to the invention. Instead, a substituate pump which does notproduce pressure waves can be used to convey the substitution fluid.This is one advantage of this embodiment.

To uncouple the dialyser 1 from the fluid system 5B, anelectromagnetically operable shut-off device 25, 26 is providedrespectively in the dialysis fluid feed line 11 upstream of the dialyser1 and in the dialyser fluid discharge line 12 downstream of the dialyser1.

The blood pump 10, the substituate pump 22 and the shut-off devices 21,25 and 26 are connected via control lines 10′, 22′, 21′, 25′ and 26′with a central control unit 27 by means of which the individualcomponents are controlled taking the predetermined treatment parametersinto consideration.

To operate the hemo(dia)filtration device as a hemodialysis device, theshut-off device 21 is closed causing the dialysis fluid to flow throughthe dialysis fluid chamber 4 of the dialyser. To operate thehemo(dia)filtration device as a hemo(dia)filtration device, the shut-offdevice 21 is opened such that sterile dialysis fluid flows from thesterile filter 16 as substitution fluid optionally into the arterialdrip chamber 8 (predilution) or the venous drip chamber 9(postdilution). However, it is also possible to operate thehemo(dia)filtration device merely as a hemofiltration device if thesupply of dialysis fluid to the dialysis fluid chamber 4 of the dialyseris interrupted. To interrupt the fluid supply, the shut-off device 25 isclosed upstream of the dialyser.

The facility for the detection of predilution and postdilution isequipped with a control unit which forms part of the central controlunit 27 of the blood treatment device. In addition, the detectionfacility is equipped with a measuring unit 28 for measuring the pressurein the extracorporeal blood stream downstream of the dialyser 1 and withan evaluation unit 29. A pressure sensor 28 fitted to the drip chamber 9is provided as measuring unit, which sensor produces an electricalpressure signal as a function of the venous pressure. The pressuresignal of the pressure sensor 28 is passed via a data transmission line30 to the evaluation unit 29 which in turn is connected via a datatransmission line 31 with the control unit 27. The evaluation unit 29 isequipped with a low-pass filter 29 a for filtering the pressure signalof the pressure sensor 28 and with a comparator 29 b by means of whichthe pressure signal is compared with a predetermined upper and lowerlimit value.

In the following, the detection of the supply of substitution fluidupstream or downstream of the dialyser is described in detail. Thecontrol unit 27 of the detection facility shuts off the substituate pump22 which is already running, preferably at the beginning of the bloodtreatment, in order to initiate the measurement. On completion of apredetermined time interval, the control unit 27 starts up thesubstituate pump 22 again. While the substituate pump is shut off, thepressure sensor 28 measures the venous pressure. The venous pressuresignal of the sensor 28 filtered by means of the low-pass filter 29 a iscompared in the comparator 29 b of the evaluation unit 29 with an upperand lower limit value in order to detect either an increase in pressureafter the shutting off of the substituate pump and a pressure drop afterthe start up of the substituate pump, or a decreases in pressure afterthe shutting off of the substituate pump and an increase in pressureafter the start up of the substituate pump.

FIG. 2A shows the basic change in the venous pressure over time in thecase of postdilution, and FIG. 2B shows the change in pressure over timein the case of predilution. It can be clearly seen that the pressureinitially increases in the case of post-dilution in order to decreasesubsequently. In the case of predilution, the pressure decreases atfirst in order to increase subsequently. On the basis of these twocharacteristic changes, the evaluation unit recognizes whetherpostdilution or predilution is present. The result can be indicatedoptically and/or acoustically on a display unit which is notillustrated.

FIGS. 3A and 3B show the venous pressure as a function of time in thecase that the predetermined time interval during which the substituatepump is shut off is relatively long. If a shorter time interval isprovided, the maximums and/or minimums lie closer together. To initiatethe measurement, the substituate pump which has already been started upis shut-off for a predetermined time interval. In principle, however, itis also possible to start up the substituate pump which had initiallybeen shut-off, for a predetermined time interval.

The characteristic change in the pressure signal shown in FIGS. 2A and2B is attributable to the changed viscosity of the blood duringpredilution or postdilution. In the case of postdilution, the viscosityof the blood is reduced as a result of the supply of substitution fluidin the venous blood line 7 downstream of the drip chamber 9. When thesubstituate pump 22 is shut-off, the viscosity of the blood increases inthis section of the venous blood line. Consequently, an increasedpressure drop occurs on the venous needle. As a result, the venouspressure increases. If the substituate pump is restarted, the viscosityof the blood in the venous blood line 7 downstream of the drip chamber 9decreases such that the venous pressure drops. As a result, a pressurepulse with a positive sign occurs following the stopping of thesubstituate pump and a pressure pulse with a negative sign occurs afterthe start up of the pump (FIG. 2A). The pulse width depends on the ratioof the blood volume between the dialysis fluid chamber 4 of the dialyser1 and the drip chamber 9 as well as the blood flow. In the case ofpredilution, on the other hand, the sequence of the signs of thepressure pulse is reversed following the shutting off and the startingup of the substituate pump. In the case of the stopping of the pump, apressure drop occurs and a pressure increase following the start up ofthe pump (FIG. 2B).

FIGS. 3A and 3B show the average venous pressure (mmHg) as a function ofthe time during an in vitro HDF dialysis treatment, the blood flow beingadjusted to 250 ml/min, the substitution rate for predilution andpostdilution to 70 ml/min and the rate of ultrafiltration to 0 ml/h. Inpractice, it has been found that the amplitudes of the pressure increaseor pressure drop differ from each other. However, the pressure increaseor pressure decrease is clearly recognizable both in the case ofpredilution and postdilution. The predetermined upper and lower limitvalues must be set such that they are below or above the maximums orminimums. On the other hand, however, the upper and lower limit valuesshould be above or below the pressure variations which are superimposedover the pressure signal.

On the basis of the positive and/or negative pressure pulses aconclusion can also be drawn regarding the fistula circulation. Arepetition of the pressure pulse sequence which can be detected in thevenous blood line 7 with the venous pressure sensor 28, and in thearterial blood line 6 which can be detected with the arterial pressuresensor, points towards a recirculation. The ratio between the integralof the pressure signal of the arterial pressure sensor in apredetermined time interval in which the repeating negative and/orpositive pressure pulse occurs and the integral of the pressure signalof the venous pressure sensor 28 in a predetermined time interval inwhich the negative and/or positive pressure pulse occurs is used as ameasure of the recirculation. The recirculation Rec [%] is calculatedaccording to the following equation:

${{Rec}\mspace{14mu}\lbrack\%\rbrack} = {100 \cdot \left( \frac{\int{P_{art}{t}}}{\int{P_{ven}{t}}} \right)}$

It is possible to draw a conclusion from the time difference (Δt)between the beginning and the end of the pressure pulse increase or thepressure pulse decrease, if the blood feed rate (Q_(b)) is known,regarding the volume of fill (V_(PD)) of the hose segment between theinlet 4 a of the dialysis fluid chamber 4 of the dialyser 1 and theconnecting site of the substitution fluid line 17, 20 to the dripchamber 9. The volume can be calculated according to the followingequation:

V _(PD) =Q _(b) Δt=Q _(b)(t _(End) −t _(Beginning))

In addition, it is possible to draw a conclusion regarding the volume offill (V_(PD)) of the hose segment between the inlet 4 a of the dialysisfluid chamber 4 and the venous needle if, in the case of HDFpredilution, the predetermined time interval between the shut-off of thesubstituate pump 22 and the beginning of the pressure drop and/or thetime interval between the start up of the substituate pump and thebeginning of the pressure increase is multiplied by the blood feed rate(Q_(b)). In the case of HDF postdilution, an analogous connectionexists, the pressure difference between the stoppage of the substituatepump and the beginning of the pressure pulse increase and/or the timedifference between the start of the substituate pump and the beginningof the pressure pulse drop being monitored.

In a converse manner, the true blood flow can be determined if thevolume of fill of the hose segments mentioned above and the timedifferences are known respectively.

If the connection between the hematocrit, the blood pressure drop and/orthe blood pressure increase, the blood feed rate and needle geometry areknown, the amplitude of the pressure increase and/or decrease providesinformation on the type of needle. If the type of needle is known, thecorresponding conclusion can be drawn regarding the hematocrit.

In this way, the change in the hematocrit can be monitored during thedialysis treatment by way of the change in the amplitude of the pressurepulses.

FIG. 4 shows an alternative embodiment of the blood treatment deviceaccording to the invention in a simplified diagrammatic representation.With the exception of the facility for the detection of predilution orpostdilution, the blood treatment device of FIG. 4 does not differ fromthe blood treatment device of FIG. 1. For this reason, the componentscorresponding to each other are indicated by the same reference signs.

The detection facility is also equipped with a venous pressure sensor 28for measuring the pressure in the venous blood line 7. The evaluationunit 50 receives the venous pressure signal of the pressure sensor 28via the data transmission line 30. The evaluation unit 50 is equippedwith a facility for the Fourier analysis 50 a which carries out aFourier analysis of the venous pressure signal.

The process for the detection of predilution or postdilution presupposesthat the substituate pump 22 and the pressure pump 10 are pumpsproducing pressure waves such as occlusion pumps, in particular rollerpumps. Since the frequency of the pressure waves produced by the bloodpump 10 is very much higher, as a result of the very much higher speedof the blood pump in comparison with the substituate pump, than thefrequency of the pressure waves of the sub stituate pump, the pressurewaves of the blood pump can be distinguished from those of the substituate pump. In the case of predilution, the pressure waves of thesubstituate pump 22 pass through not only the hose section of thesubstitution fluid line 17, 19 but also the hose section of the arterialblood line 6 between the drip chamber 8 and the inlet 3 a of the bloodfluid chamber 3, the blood fluid chamber and the venous blood line 7before reaching the pressure sensor 28. In the case of postdilution, thepressure waves, on the other hand, do not pass through the blood fluidchamber and the corresponding blood line sections. The effect of theblood fluid chamber 3 and the associated blood line sections can bedescribed as being a transmission function. When the dialyser and theassociated line sections are situated in the transmission path of thepressure waves, the dynamics of the pressure pulses and their frequencyspectrum change, higher frequencies, in particular, being more stronglyattenuated.

An accurate knowledge of the transmission function is not necessary forthe detection of predilution or postdilution. It is sufficient if arelationship is established between the pressure signal of thesubstituate pump and the pressure signal of the blood pump.

The pressure signal measured with the pressure sensor 28 contains boththe pressure signal of the blood pump 10 and that of the substituatepump 22. The facility for the Fourier transformation 50 a of theevaluation unit 50 breaks down the pressure signal of the blood pump 10and the substituate pump 22 measured with the pressure sensor 28 intothe signal components attributable to the blood pump and/or thesubstituate pump.

FIGS. 5A and 5B show the frequency spectrum of the venous pressuresignal determined by the Fourier transformation in the case ofpostdilution or predilution. The measured values shown in the figureswere determined in a laboratory test during which the blood flow wasadjusted to 300 ml/min, the substituate flow to 80 ml/min and theultrafiltration rate to 100 ml/h. In the frequency spectrum, thecoefficients of the first harmonic component and the harmonic componentsof a higher order can be recognized which are attributable to the bloodpump 10 and the substituate pump 22.

The evaluation unit 50 calculates the quotient between a harmonic of ahigher order, e.g. the second or third harmonic and the coefficient ofthe harmonic of the first order, i.e. the standardized coefficients ofspectral splitting. The standardized coefficient of the substituate pumprepresents a first evaluation factor whereas the standardizedcoefficient of the blood pump represents a second evaluation factor. Forcomparing the evaluation factors of the substituate pump and the bloodpump, the evaluation unit 50 is equipped with a further unit 50 b. Ifthe evaluation factor of the substituate pump lies substantially abovethe evaluation factor of the blood pump, the conclusion is drawn thatpostdilution is present (FIG. 5A). If the evaluation factor of thesubstituate pump is merely above or in the vicinity of the substituatepump or if both evaluation factors are the same, the conclusion is drawnthat predilution is present (FIG. 5B). The detection of predilution orpostdilution on the basis of the comparison of the evaluation factorstakes place in a facility 50 c.

The facility 50 c for recognizing predilution or postdilution isequipped with a facility for forming the difference between the twoevaluation factors. The difference is compared with a predeterminedlimit value which is determined such that a reliable distinction can bemade between predilution and postdilution. A postdilution is present ifthe difference between the first and the second evaluation factor isgreater than the predetermined limit value and a predilution is presentif the difference between the first and the second evaluation factor isless than the predetermined limit value. The result can be indicated inan optical and/or acoustic display unit which is not illustrated.

What is claimed is:
 1. A method of determining fistula recirculation fora device for the extracorporeal treatment of blood, wherein the devicefor the extracorporeal treatment of blood comprises an extracorporealblood circuit including a first chamber of a dialyser or a filter, and afluid system including a second chamber of the dialyser or the filter,wherein the dialyser or the filter is divided by a semipermeablemembrane into the first chamber and the second chamber, and whereinsubstitution fluid is supplied to the extracorporeal blood circuitupstream or downstream of the dialyser or the filter by means of asubstituate pump, the method comprising: at least one of starting up thesubstituate pump which has been previously shut off, or shutting off thesubstituate pump which has been previously started up; measuring apressure in the extracorporeal blood circuit upstream of the dialyser orthe filter and downstream of the dialyser or the filter after the atleast one of starting up the sub stituate pump which has been previouslyshut off, or shutting off the substituate pump which has been previouslystarted up; and determining fistula recirculation by calculating theratio between the integral of the pressure measured upstream of thedialyser or the filter in a predetermined time interval in which arepeating positive and/or negative pressure pulse occurs, and theintegral of the pressure measured downstream of the dialyser or thefilter in a predetermined time interval in which a repeating positiveand/or negative pressure pulse occurs.
 2. The method according to claim1, wherein the pressure in the extracorporeal blood circuit is measuredwith an arterial pressure sensor upstream of the dialyser or the filterand a venous pressure sensor downstream of the dialyser or the filter.3. The method according to claim 2, wherein a pressure signal of atleast one of the arterial pressure sensor or the venous pressure sensoris filtered with a low-pass filter.
 4. A device for the extracorporealtreatment of blood, the device comprising: a dialyser or a filterdivided by a semipermeable membrane into a first chamber and a secondchamber; an extracorporeal blood circuit including the first chamber ofthe dialyser or the filter; a fluid system including the second chamberof the dialyser or the filter; a substitutate pump connected to asubstitution fluid line leading to the extracorporeal blood circuitupstream or downstream of the dialyser or the filter; a control unitconfigured to at least one of start up the substituate pump or shut offthe substituate pump; a measuring unit for measuring a pressure in theextracorporeal blood circuit upstream of the dialyser or the filter anddownstream of the dialyser or the filter; and an evaluation unitconfigured to determine fistula recirculation by calculating the ratiobetween the integral of the pressure measured upstream of the dialyseror the filter in a predetermined time interval in which a repeatingpositive and/or negative pressure pulse occurs, and the integral of thepressure measured downstream of the dialyser or the filter in apredetermined time interval in which a repeating positive and/ornegative pressure pulse occurs.
 5. The device according to claim 4,wherein the measuring unit for measuring the pressure in theextracorporeal blood circuit upstream and downstream of the dialyser orthe filter comprises an arterial pressure sensor in an arterial bloodline of the extracorporeal blood circuit and a venous pressure sensor inthe venous blood line of the extracorporeal blood circuit.
 6. The deviceaccording to claim 5, wherein the measuring unit further comprises alow-pass filter connected to the signal output end of at least one ofthe arterial pressure sensor or the venous pressure sensor.